tests: make test-extension.t's baduisetup.py test fail even if bdiff exists
A mercurial.bdiff module has existed at some point and when I have
built at an old version, some .pyc file or something seems to stick
around and test-extension.t fails. Let's remove the reference to bdiff
and simplify the test.
Differential Revision: https://phab.mercurial-scm.org/D1162
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a